diff --git a/lib/main.dart b/lib/main.dart
index ca6f4a1..553fcc2 100644
--- a/lib/main.dart
+++ b/lib/main.dart
@@ -1,6 +1,8 @@
+import 'dart:async';
 import 'dart:math' as math;
 import 'dart:typed_data';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_colorpicker/flutter_colorpicker.dart';
 import 'package:image/image.dart' as img;
@@ -32,13 +34,18 @@ class MosaicHomePage extends StatefulWidget {
 
 class _MosaicHomePageState extends State<MosaicHomePage> {
   final ImagePicker _picker = ImagePicker();
-  final TextEditingController _gridWidthCtrl = TextEditingController(text: '40');
-  final TextEditingController _gridHeightCtrl = TextEditingController(text: '30');
+  final TextEditingController _gridWidthCtrl =
+      TextEditingController(text: '40');
+  final TextEditingController _gridHeightCtrl =
+      TextEditingController(text: '30');
   final TextEditingController _capSizeCtrl = TextEditingController(text: '12');
 
   Uint8List? _sourceImageBytes;
   MosaicResult? _result;
   bool _useCapSize = false;
+  bool _isGenerating = false;
+  Timer? _debounceTimer;
+  int _generationToken = 0;
 
   final List<CapColor> _palette = [
     CapColor(name: 'White', color: const Color(0xFFF2F2F2)),
@@ -48,8 +55,17 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
     CapColor(name: 'Green', color: const Color(0xFF4FAE63)),
   ];
 
+  @override
+  void initState() {
+    super.initState();
+    _gridWidthCtrl.addListener(_scheduleRegenerate);
+    _gridHeightCtrl.addListener(_scheduleRegenerate);
+    _capSizeCtrl.addListener(_scheduleRegenerate);
+  }
+
   @override
   void dispose() {
+    _debounceTimer?.cancel();
     _gridWidthCtrl.dispose();
     _gridHeightCtrl.dispose();
     _capSizeCtrl.dispose();
@@ -66,10 +82,19 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
     });
   }
 
+  void _scheduleRegenerate() {
+    if (_sourceImageBytes == null || _result == null) return;
+    _debounceTimer?.cancel();
+    _debounceTimer = Timer(const Duration(milliseconds: 300), _generate);
+  }
+
   void _addCapDialog() {
     Color selected = Colors.orange;
     final nameCtrl = TextEditingController();
-    final hexCtrl = TextEditingController(text: '#${selected.toARGB32().toRadixString(16).substring(2).toUpperCase()}');
+    final hexCtrl = TextEditingController(
+      text:
+          '#${selected.toARGB32().toRadixString(16).substring(2).toUpperCase()}',
+    );
 
     showDialog<void>(
       context: context,
@@ -98,22 +123,30 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
                   pickerColor: selected,
                   onColorChanged: (c) {
                     selected = c;
-                    hexCtrl.text = '#${c.toARGB32().toRadixString(16).substring(2).toUpperCase()}';
+                    hexCtrl.text =
+                        '#${c.toARGB32().toRadixString(16).substring(2).toUpperCase()}';
                   },
                 ),
               ],
             ),
           ),
           actions: [
-            TextButton(onPressed: () => Navigator.pop(ctx), child: const Text('Cancel')),
+            TextButton(
+              onPressed: () => Navigator.pop(ctx),
+              child: const Text('Cancel'),
+            ),
             FilledButton(
               onPressed: () {
                 final name = nameCtrl.text.trim();
                 if (name.isEmpty) return;
                 setState(() {
-                  _palette.add(CapColor(name: name, color: _parseHex(hexCtrl.text) ?? selected));
+                  _palette.add(
+                    CapColor(
+                        name: name, color: _parseHex(hexCtrl.text) ?? selected),
+                  );
                 });
                 Navigator.pop(ctx);
+                _scheduleRegenerate();
               },
               child: const Text('Add'),
             ),
@@ -123,52 +156,58 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
     );
   }
 
-  void _generate() {
+  Future<void> _generate() async {
     if (_sourceImageBytes == null || _palette.isEmpty) return;
-    final decoded = img.decodeImage(_sourceImageBytes!);
-    if (decoded == null) return;
 
-    final int gridW;
-    final int gridH;
-    if (_useCapSize) {
-      final capSize = math.max(1, int.tryParse(_capSizeCtrl.text) ?? 12);
-      gridW = math.max(1, (decoded.width / capSize).round());
-      gridH = math.max(1, (decoded.height / capSize).round());
-    } else {
-      gridW = math.max(1, int.tryParse(_gridWidthCtrl.text) ?? 40);
-      gridH = math.max(1, int.tryParse(_gridHeightCtrl.text) ?? 30);
-    }
+    final int gridW = math.max(1, int.tryParse(_gridWidthCtrl.text) ?? 40);
+    final int gridH = math.max(1, int.tryParse(_gridHeightCtrl.text) ?? 30);
+    final int capSize = math.max(1, int.tryParse(_capSizeCtrl.text) ?? 12);
 
-    final scaled = img.copyResize(decoded, width: gridW, height: gridH, interpolation: img.Interpolation.average);
-    final paletteLab = _palette.map((p) => _rgbToLab(p.color)).toList();
+    final token = ++_generationToken;
+    setState(() => _isGenerating = true);
 
-    final List<int> assignments = List<int>.filled(gridW * gridH, 0);
+    final payload = <String, dynamic>{
+      'source': _sourceImageBytes!,
+      'useCapSize': _useCapSize,
+      'gridW': gridW,
+      'gridH': gridH,
+      'capSize': capSize,
+      'palette': _palette
+          .map((p) =>
+              <String, dynamic>{'name': p.name, 'value': p.color.toARGB32()})
+          .toList(growable: false),
+    };
+
+    final out = await compute(_generateMosaicIsolate, payload);
+    if (!mounted || token != _generationToken) return;
+
+    final palette = (out['palette'] as List)
+        .map(
+          (e) => CapColor(
+              name: e['name'] as String, color: Color(e['value'] as int)),
+        )
+        .toList(growable: false);
+
+    final countsList = (out['counts'] as List).cast<int>();
     final counts = <CapColor, int>{};
-
-    for (int y = 0; y < gridH; y++) {
-      for (int x = 0; x < gridW; x++) {
-        final pix = scaled.getPixel(x, y);
-        final srcColor = Color.fromARGB(255, pix.r.toInt(), pix.g.toInt(), pix.b.toInt());
-        final srcLab = _rgbToLab(srcColor);
-
-        int bestIdx = 0;
-        double bestDistance = double.infinity;
-        for (int i = 0; i < _palette.length; i++) {
-          final d = _deltaE76(srcLab, paletteLab[i]);
-          if (d < bestDistance) {
-            bestDistance = d;
-            bestIdx = i;
-          }
-        }
-
-        assignments[y * gridW + x] = bestIdx;
-        final cap = _palette[bestIdx];
-        counts[cap] = (counts[cap] ?? 0) + 1;
-      }
+    for (int i = 0; i < palette.length; i++) {
+      counts[palette[i]] = countsList[i];
     }
 
+    final sortedCounts = counts.entries.toList()
+      ..sort((a, b) => b.value.compareTo(a.value));
+
     setState(() {
-      _result = MosaicResult(width: gridW, height: gridH, assignments: assignments, palette: List.of(_palette), counts: counts);
+      _result = MosaicResult(
+        width: out['width'] as int,
+        height: out['height'] as int,
+        assignments: (out['assignments'] as List).cast<int>(),
+        palette: palette,
+        counts: counts,
+        sortedCounts: sortedCounts,
+        previewPng: out['previewPng'] as Uint8List,
+      );
+      _isGenerating = false;
     });
   }
 
@@ -177,9 +216,15 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
     return Scaffold(
       appBar: AppBar(title: const Text('Bottle-Cap Mosaic Prototype')),
       floatingActionButton: FloatingActionButton.extended(
-        onPressed: _generate,
-        icon: const Icon(Icons.auto_fix_high),
-        label: const Text('Generate Mosaic'),
+        onPressed: _isGenerating ? null : _generate,
+        icon: _isGenerating
+            ? const SizedBox(
+                width: 18,
+                height: 18,
+                child: CircularProgressIndicator(strokeWidth: 2),
+              )
+            : const Icon(Icons.auto_fix_high),
+        label: Text(_isGenerating ? 'Generating…' : 'Generate Mosaic'),
       ),
       body: Padding(
         padding: const EdgeInsets.all(12),
@@ -205,7 +250,10 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
                 ButtonSegment(value: true, label: Text('Cap size (px)')),
               ],
               selected: {_useCapSize},
-              onSelectionChanged: (s) => setState(() => _useCapSize = s.first),
+              onSelectionChanged: (s) {
+                setState(() => _useCapSize = s.first);
+                _scheduleRegenerate();
+              },
             ),
             const SizedBox(height: 8),
             if (!_useCapSize)
@@ -215,7 +263,8 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
                     child: TextField(
                       controller: _gridWidthCtrl,
                       keyboardType: TextInputType.number,
-                      decoration: const InputDecoration(labelText: 'Grid Width'),
+                      decoration:
+                          const InputDecoration(labelText: 'Grid Width'),
                     ),
                   ),
                   const SizedBox(width: 8),
@@ -223,7 +272,8 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
                     child: TextField(
                       controller: _gridHeightCtrl,
                       keyboardType: TextInputType.number,
-                      decoration: const InputDecoration(labelText: 'Grid Height'),
+                      decoration:
+                          const InputDecoration(labelText: 'Grid Height'),
                     ),
                   ),
                 ],
@@ -232,13 +282,23 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
               TextField(
                 controller: _capSizeCtrl,
                 keyboardType: TextInputType.number,
-                decoration: const InputDecoration(labelText: 'Approx cap size in source image (pixels)'),
+                decoration: const InputDecoration(
+                  labelText: 'Approx cap size in source image (pixels)',
+                ),
               ),
             const SizedBox(height: 16),
             Row(
               children: [
-                const Expanded(child: Text('Cap Palette', style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold))),
-                IconButton(onPressed: _addCapDialog, icon: const Icon(Icons.add_circle_outline)),
+                const Expanded(
+                  child: Text(
+                    'Cap Palette',
+                    style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
+                  ),
+                ),
+                IconButton(
+                  onPressed: _addCapDialog,
+                  icon: const Icon(Icons.add_circle_outline),
+                ),
               ],
             ),
             Wrap(
@@ -249,35 +309,47 @@ class _MosaicHomePageState extends State<MosaicHomePage> {
                   Chip(
                     avatar: CircleAvatar(backgroundColor: _palette[i].color),
                     label: Text(_palette[i].name),
-                    onDeleted: _palette.length <= 1 ? null : () => setState(() => _palette.removeAt(i)),
+                    onDeleted: _palette.length <= 1
+                        ? null
+                        : () {
+                            setState(() => _palette.removeAt(i));
+                            _scheduleRegenerate();
+                          },
                   ),
               ],
             ),
             const SizedBox(height: 16),
             if (_result != null) ...[
-              Text('Preview (${_result!.width} x ${_result!.height})', style: const TextStyle(fontSize: 18, fontWeight: FontWeight.bold)),
+              Text(
+                'Preview (${_result!.width} x ${_result!.height})',
+                style:
+                    const TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
+              ),
               const SizedBox(height: 8),
-              AspectRatio(
-                aspectRatio: _result!.width / _result!.height,
-                child: CustomPaint(
-                  painter: MosaicPainter(_result!),
+              RepaintBoundary(
+                child: AspectRatio(
+                  aspectRatio: _result!.width / _result!.height,
+                  child: Image.memory(
+                    _result!.previewPng,
+                    fit: BoxFit.fill,
+                    filterQuality: FilterQuality.none,
+                    gaplessPlayback: true,
+                  ),
                 ),
               ),
               const SizedBox(height: 16),
-              const Text('Bill of Materials', style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold)),
-              ...(() {
-                final items = _result!.counts.entries.toList()..sort((a, b) => b.value.compareTo(a.value));
-                return items
-                    .map(
-                      (e) => ListTile(
-                        dense: true,
-                        leading: CircleAvatar(backgroundColor: e.key.color),
-                        title: Text(e.key.name),
-                        trailing: Text('${e.value} caps'),
-                      ),
-                    )
-                    .toList();
-              })(),
+              const Text(
+                'Bill of Materials',
+                style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
+              ),
+              ..._result!.sortedCounts.map(
+                (e) => ListTile(
+                  dense: true,
+                  leading: CircleAvatar(backgroundColor: e.key.color),
+                  title: Text(e.key.name),
+                  trailing: Text('${e.value} caps'),
+                ),
+              ),
             ],
           ],
         ),
@@ -298,7 +370,7 @@ class CapColor {
   final String name;
   final Color color;
 
-  CapColor({required this.name, required this.color});
+  const CapColor({required this.name, required this.color});
 }
 
 class MosaicResult {
@@ -307,38 +379,130 @@ class MosaicResult {
   final List<int> assignments;
   final List<CapColor> palette;
   final Map<CapColor, int> counts;
+  final List<MapEntry<CapColor, int>> sortedCounts;
+  final Uint8List previewPng;
 
-  MosaicResult({required this.width, required this.height, required this.assignments, required this.palette, required this.counts});
+  MosaicResult({
+    required this.width,
+    required this.height,
+    required this.assignments,
+    required this.palette,
+    required this.counts,
+    required this.sortedCounts,
+    required this.previewPng,
+  });
 }
 
-class MosaicPainter extends CustomPainter {
-  final MosaicResult result;
+Map<String, dynamic> _generateMosaicIsolate(Map<String, dynamic> request) {
+  final source = request['source'] as Uint8List;
+  final useCapSize = request['useCapSize'] as bool;
+  final defaultGridW = request['gridW'] as int;
+  final defaultGridH = request['gridH'] as int;
+  final capSize = request['capSize'] as int;
+  final paletteRaw = (request['palette'] as List).cast<Map>();
 
-  MosaicPainter(this.result);
+  final decoded = img.decodeImage(source);
+  if (decoded == null) {
+    return <String, dynamic>{
+      'width': 1,
+      'height': 1,
+      'assignments': <int>[0],
+      'counts': <int>[1],
+      'palette': paletteRaw,
+      'previewPng': Uint8List.fromList(
+        img.encodePng(img.Image(width: 1, height: 1)),
+      ),
+    };
+  }
 
-  @override
-  void paint(Canvas canvas, Size size) {
-    final cellW = size.width / result.width;
-    final cellH = size.height / result.height;
-    final paint = Paint();
+  final gridW = useCapSize
+      ? math.max(1, (decoded.width / capSize).round())
+      : defaultGridW;
+  final gridH = useCapSize
+      ? math.max(1, (decoded.height / capSize).round())
+      : defaultGridH;
 
-    for (int y = 0; y < result.height; y++) {
-      for (int x = 0; x < result.width; x++) {
-        final idx = result.assignments[y * result.width + x];
-        paint.color = result.palette[idx].color;
-        canvas.drawRect(Rect.fromLTWH(x * cellW, y * cellH, cellW, cellH), paint);
+  final scaled = img.copyResize(
+    decoded,
+    width: gridW,
+    height: gridH,
+    interpolation: img.Interpolation.average,
+  );
+
+  final paletteValues = paletteRaw.map((e) => e['value'] as int).toList();
+  final paletteLab = paletteValues.map(_argbToLab).toList(growable: false);
+
+  final assignments = List<int>.filled(gridW * gridH, 0);
+  final counts = List<int>.filled(paletteValues.length, 0);
+
+  final nearestCache = <int, int>{};
+  final labCache = <int, List<double>>{};
+
+  for (int y = 0; y < gridH; y++) {
+    for (int x = 0; x < gridW; x++) {
+      final pix = scaled.getPixel(x, y);
+      final rgb = (pix.r.toInt() << 16) | (pix.g.toInt() << 8) | pix.b.toInt();
+
+      final cached = nearestCache[rgb];
+      if (cached != null) {
+        assignments[y * gridW + x] = cached;
+        counts[cached]++;
+        continue;
       }
+
+      final srcLab = labCache.putIfAbsent(rgb, () => _rgbToLab(rgb));
+
+      int bestIdx = 0;
+      double bestDistance = double.infinity;
+      for (int i = 0; i < paletteLab.length; i++) {
+        final d = _deltaE76(srcLab, paletteLab[i]);
+        if (d < bestDistance) {
+          bestDistance = d;
+          bestIdx = i;
+        }
+      }
+
+      nearestCache[rgb] = bestIdx;
+      assignments[y * gridW + x] = bestIdx;
+      counts[bestIdx]++;
     }
   }
 
-  @override
-  bool shouldRepaint(covariant MosaicPainter oldDelegate) => oldDelegate.result != result;
+  final preview = img.Image(width: gridW, height: gridH);
+  for (int y = 0; y < gridH; y++) {
+    for (int x = 0; x < gridW; x++) {
+      final idx = assignments[y * gridW + x];
+      final argb = paletteValues[idx];
+      preview.setPixelRgba(
+        x,
+        y,
+        (argb >> 16) & 0xFF,
+        (argb >> 8) & 0xFF,
+        argb & 0xFF,
+        255,
+      );
+    }
+  }
+
+  return <String, dynamic>{
+    'width': gridW,
+    'height': gridH,
+    'assignments': assignments,
+    'counts': counts,
+    'palette': paletteRaw,
+    'previewPng': Uint8List.fromList(img.encodePng(preview, level: 1)),
+  };
 }
 
-List<double> _rgbToLab(Color c) {
-  double r = c.r / 255.0;
-  double g = c.g / 255.0;
-  double b = c.b / 255.0;
+List<double> _argbToLab(int argb) {
+  final rgb = argb & 0x00FFFFFF;
+  return _rgbToLab(rgb);
+}
+
+List<double> _rgbToLab(int rgb) {
+  double r = ((rgb >> 16) & 0xFF) / 255.0;
+  double g = ((rgb >> 8) & 0xFF) / 255.0;
+  double b = (rgb & 0xFF) / 255.0;
 
   r = r <= 0.04045 ? r / 12.92 : math.pow((r + 0.055) / 1.055, 2.4).toDouble();
   g = g <= 0.04045 ? g / 12.92 : math.pow((g + 0.055) / 1.055, 2.4).toDouble();
@@ -348,7 +512,9 @@ List<double> _rgbToLab(Color c) {
   final y = (r * 0.2126 + g * 0.7152 + b * 0.0722) / 1.00000;
   final z = (r * 0.0193 + g * 0.1192 + b * 0.9505) / 1.08883;
 
-  double f(double t) => t > 0.008856 ? math.pow(t, 1.0 / 3.0).toDouble() : (7.787 * t) + (16 / 116);
+  double f(double t) => t > 0.008856
+      ? math.pow(t, 1.0 / 3.0).toDouble()
+      : (7.787 * t) + (16 / 116);
 
   final fx = f(x);
   final fy = f(y);